Acute respiratory infection (ARI) remains a major source of morbidity and mortality worldwide. Epidemiologic evidence and experience with three vaccines targeting ARIs point to the acquisition of colonization as the key event in eventual progression to disease, transmission and prevention. This application focuses on understanding the first event in host-pathogen interaction, establishment of colonization on the mucosal surface of the upper respiratory tract (URT). Streptococcus pneumoniae, a leading cause of ARI particularly in the setting of viral co- infection, will be used to examine the microbial and host factors contributing to acquisition. Preliminary data show that prior to the establishment of a stable bacterial population along the epithelial surface pneumococci are found entrapped in lumenal mucus. We will use a mouse model of carriage to examine pneumococcal interaction with mucus and how this earliest event dictates success in colonization (Aim#1) and is affected by inflammation (Aim#2) and immunity (Aim#3). Preliminary data support a mechanism to be tested in Aim#1 whereby the ability of the pneumococcus to manipulate sugars allows it to breakdown and utilize mucopolysaccharides for its proliferation, adaptation and to escape mucus entrapment. We propose that these events are required for the development of colonization of sufficient density and duration to allow for transmission. Additional preliminary data suggest that URT inflammation promotes rather than inhibits pneumococcal colonization. The role of pneumococcal-induced inflammation and co- infection with influenza A on mucus production and their role in enhancing early events in colonization will be examined in Aim#2. In Aim#3 we will examine the contribution of IgG from the serum pool in blocking acquisition and transmission, and determine the mechanism for antibody-mediated inhibition of early events in colonization. Thus, this application addresses basic processes of broad significance to pathogenesis- i) acquisition of a microbe by a host, ii) proliferation in that host, iii) transmisson to a new host, iv) the effect of inflammation in the microbe's niche, and v) the modulation of these parameters in an immune host.